“It hasn’t crossed my mind to ever want to leave the city,” said the 25-year-old Mai, who shares the 450-square-foot apartment with her boyfriend. “I’ve never thought of our place as too small. I really don’t need a big kitchen or a garden.”

Mai is among the thousands of young professionals whose devotion to urban living is causing Boston to grow at its fastest rate in decades. The influx has spawned a sweeping transformation of the city, with new residences and office buildings filling the skyline and reinventing commercial districts that once felt hopelessly time-worn.

Almost everywhere you look, it seems, is a new building site: A dozen towers are rising in the downtown area, and city-wide some 5,300 homes are currently under development. Boylston Street near Fenway Park is humming with construction during the day and crowds of diners at night. Downtown Crossing has lured fine restaurants and hundreds of luxury residences. And even once rough-hewn neighborhoods such as South Boston are increasingly drawing gourmet food stores, hip bars, and tony apartments.

The population surge has thoroughly reversed the suburban migration that began in the 1950s, when Boston peaked at about 800,000 people. Head counts in the South End and downtown have jumped by 20 percent since 2000.

In just one year alone — 2010 — Boston’s population grew by 7,500 people, and is now above 625,000, its highest level since the 1970s, according to city data.

Though largely driven by the generation between 20 and 34, the city is also swelling as empty-nesters trade suburban homes for urban pied-a-terres, and more young families opt for Boston over the traditional move to the suburbs in search of better schools. Regardless of background and interests, these people are drawn by the convenience and energy of busy urban neighborhoods.

“I like the feeling of being surrounded by people,” said Ece Gulsen, 27, who grew up on Turkey’s Mediterranean coast and now lives in the Charlestown Navy Yard to be near the water.

Companies are also moving into Boston to attract talented workers, developers are responding with even more housing, and restaurateurs, sensing a growing appetite for inventive food and entertainment, are opening eateries in places that defy conventional wisdom.

During the last two decades, Brad Fredericks, proprietor of Fajitas & ’Ritas, has watched the changes wash through the city’s downtown, where he recently opened a new eatery, the Back Deck grill on West Street. First, he said, Suffolk University and Emerson College expanded. Then the Boston Opera House was renovated, and the Paramount and Modern theatres reopened. Businesses then formed an association to help improve the area, and hundreds of apartments and condominiums are now under construction.

“I’m more optimistic than I’ve ever been,” he said. “There have always been a lot of activity generators down here. But the crowds have become more consistent. You see more people strolling around.”

The Boston real estate market is one of the strongest in the country, according to the Urban Land Institute, in part because of its strong housing market and the medical and technology companies who want to be near its population of highly educated 20- to 34-year-olds.

Maureen McAvey, a ULI fellow who specializes in retail development, said young professionals have particular preferences for housing, shopping, and travel that dictate how a city grows. For one, they are more willing to live in small spaces. They don’t feel the need to own a car, and make more frequent shopping trips.

“From a consumer standpoint, we’ve seen a large increase in people buying food on a two- to three-day basis,” McAvey said. “This generation wants the access and convenience that the city provides. They are much less interested in having a big lawn.”

But it is not just young workers queuing up for the city. Dick Reynolds, 67, relocated with his wife to a two-bedroom condominium in the South End when their kids moved out of their old home in Needham.

“We’re delighted with it,” he said. “We’ve always loved the ambiance of the city. And we can go to a variety of things without getting into the car. We don’t have to worry about parking, cutting the grass, or shoveling snow.”

Though positive in many respects, the population growth creates many challenges for city officials and residents alike: crowded schools, roads, and transit lines, and harder-to-find housing at moderate prices.

“It’s virtually impossible for someone of my income level to own or rent in the city,” said Quinton Kerns, a 27-year-old urban designer who pays $600 a month to share a Harvard Square apartment with five roommates. And with $150,000 in student loan debt, Kerns doesn’t see himself moving up in the housing market anytime soon.

“It’s frustrating — I can’t just go to the community of my choosing,” he said. “I’m at the mercy of what’s affordable to me.”

Even though Boston added more units of housing in the last decade than in the three previous decades combined, the pace of new development is not keeping up with all the people who want to live here. The Dukakis Center for Urban and Regional Policy at Northeastern University predicts that unless annual housing production in the Boston area doubles — from 6,000 units to 12,000 units a year — already sky-high prices will soar.

“The circumstances call for a very different approach to housing,” Dukakis center director Barry Bluestone said. “There’s much more demand for multifamily rentals and condos. We need to get communities to rezone to allow for that kind of housing.”

Average rents in Boston are about $1,700 a month. But much of the new housing built in the past few years are luxury residences that command monthly rent of $4,000 and more. Both the city and state have launched initiatives to build more moderate-priced housing; the Compact Neighborhoods program by the Patrick administration aims to spur construction of 10,000 multifamily housing units a year in Massachusetts, largely to retain young workers being priced out of the market.

Mayor Thomas Menino’s administration has begun encouraging developers in the South Boston Innovation District to build micro-housing units — tiny apartments with rents that people just starting out can afford.

Yet here too that goal is proving elusive. At Factory 63, a newly renovated building with units as small as 375-square-feet, so many people applied for its first group of apartments that a lottery was required to parcel them out. The prices ended up at $1,700 to $2,400 a month, a few hundred dollars higher than officials had initially hoped.

“Just because the units are smaller doesn’t mean you can build them cheaply,” said Kelly Saito, president of Portland, Ore.-based developer Gerding Edlen, which built Factory 63 on Melcher Street and is also constructing a 21-story housing tower on A Street.

Boston already has among the highest construction and land acquisition costs in the country, he said, while a building full of studio-sized apartments means many more expensive kitchen and bathroom fixtures than normal.

“I think the effort to produce lower cost housing can be achieved at least partially,” Saito said. “But it really depends on where the expectations lie.”

Meanwhile Boston is grappling with another by-product of its popularity: crowded classrooms. Enrollment in city schools next year is expected to be at its highest in a decade, with another 1,200 children entering the school system, mostly at the lower grades. The additional students will require a $61 million increase to the city’s school budget, according to a recent report to the school committee.

Boston officials are in the midst of changing the classroom assignment process to make it more predictable and to provide easier access to quality neighborhood schools.

One of the fastest changing neighborhoods is the Fenway area, where the population increased 15 percent from 2000 to 2010. For decades its main boulevard — Boylston Street — was a scrubby, traffic-choked row of gas stations and repair shops. But in just a few short years, several modern, sleek apartment and retail buildings have gone up, and the strip now boasts a sushi place, Southern barbecue restaurant, and popular nightspots that spill crowds well into the night.

Dave DuBois, chief executive of the Franklin Restaurant Group, said the neighborhood’s rapid growth has quickly produced a creative food scene that is entirely distinct from Fenway Park and nearby Lansdowne Street.

“Five years ago, if I opened a restaurant in the Fenway, people would say, ‘Oh, you doing beer and nachos? Wings?,’ ” said DuBois, whose company opened Citizen Public House, a fine whiskey bar on Boylston Street that serves a pork pate melt. “The neighborhood has the energy of people in motion. Top restaurateurs are taking a serious look at it.”

In February developer Samuels & Associates proposed construction of another 22-story residential and retail building called The Point — a sharply-angled glass building that would replace a row of run-down retail buildings at Boylston and Brookline Avenue.

Mai, the Beacon Hill renter, said she would love to move into one of the new buildings rising across the city, but the prices remain stratospheric. She said she and her boyfriend are able to afford the $1,600 a month rent in their Beacon Hill unit. But her landlord has already increased the rent once, and may try do so again.

Mai is beginning to consider other options, but like many in her age group, communities outside the city are not among them.

“I think the farthest I would go is the South End,” said Mai. “I know a lot of people want to live in the suburbs because they crave the extra space, but it’s never been something I’ve wanted. I’m happy staying around the city.”